207 research outputs found
Molecular Gas in Spiral Galaxies
In this review, I highlight a number of recent surveys of molecular gas in
nearby spiral galaxies. Through such surveys, more complete observations of the
distribution and kinematics of molecular gas have become available for galaxies
with a wider range of properties (e.g., brightness, Hubble type, strength of
spiral or bar structure). These studies show the promise of both
interferometers and single-dish telescopes in advancing our general
understanding of molecular gas in spiral galaxies. In particular, I highlight
the contributions of the recent BIMA Survey of Nearby Galaxies (SONG).Comment: 8 pages, 1 figure. To appear in the proceedings of the 4th
Cologne-Bonn-Zermatt-Symposium, "The Dense Interstellar Medium in Galaxies",
which was held in Zermatt, Switzerland in September 200
[S IV] in the NGC 5253 Supernebula: Ionized Gas Kinematics at High Resolution
The nearby dwarf starburst galaxy NGC 5253 hosts a deeply embedded
radio-infrared supernebula excited by thousands of O stars. We have observed
this source in the 10.5{\mu}m line of S+3 at 3.8 kms-1 spectral and 1.4"
spatial resolution, using the high resolution spectrometer TEXES on the IRTF.
The line profile cannot be fit well by a single Gaussian. The best simple fit
describes the gas with two Gaussians, one near the galactic velocity with FWHM
33.6 km s-1 and another of similiar strength and FWHM 94 km s-1 centered \sim20
km s-1 to the blue. This suggests a model for the supernebula in which gas
flows towards us out of the molecular cloud, as in a "blister" or "champagne
flow" or in the HII regions modelled by Zhu (2006).Comment: Accepted for publication in the Astrophysical Journal 4 June 201
CO(1-0), CO(2-1) and Neutral Gas in NGC 6946: Molecular Gas in a Late-Type, Gas Rich, Spiral Galaxy
We present "On The Fly" maps of the CO(1-0) and CO(2-1) emission covering a
10' X 10' region of the NGC 6946. Using our CO maps and archival VLA HI
observations we create a total gas surface density map, Sigma_gas, for NGC
6946. The predominantly molecular inner gas disk transitions smoothly into an
atomic outer gas disk, with equivalent atomic and molecular gas surface
densities at R = 3.5' (6 kpc). We estimate that the total H2 mass is 3 X 10^9
Mo, roughly 1/3 of the interstellar hydrogen gas mass, and about 2% of the
dynamical mass of the galaxy at our assumed distance of 6 Mpc. The value of the
CO(2-1)/CO(1-0) line ratio ranges from 0.35 to 2; 50% of the map is covered by
very high ratio, >1, gas. The very high ratios are predominantly from interarm
regions and appear to indicate the presence of wide-spread optically thin gas.
Star formation tracers are better correlated with the total neutral gas disk
than with the molecular gas by itself implying SFR is proportional to
Sigma_gas. Using the 100 FIR and 21 cm continuum from NGC 6946 as star
formation tracers, we arrive at a gas consumption timescale of 2.8 Gyr, which
is relatively uniform across the disk. The high star formation rate at the
nucleus appears to be due to a large accumulation of molecular gas rather than
a large increase in the star formation efficiency. The mid-plane gas pressure
in the outer (R > 10 kpc) HI arms of NGC 6946 is close to the value at the
radial limit (10 kpc) of our observed CO disk. If the mid-plane gas pressure is
a factor for the formation of molecular clouds, these outer HI gas arms should
contain molecular gas which we do not see because they are beyond our detection
limit
Nuclear Bar Catalyzed Star Formation: 13^CO, C18^O and Molecular Gas Properties in the Nucleus of Maffei 2
(Abridged) We present resolution maps of CO, its isotopologues, and HCN from
in the center of Maffei 2. The J=1-0 rotational lines of 12^CO, 13^CO, C18^O
and HCN, and the J=2-1 lines of 13^CO and C18^O were observed with the OVRO and
BIMA arrays. The 2-1/1-0 line ratios of the isotopologues constrain the bulk of
the molecular gas to originate in low excitation, subthermal gas. From LVG
modeling, we infer that the central GMCs have n(H_2) ~10^2.75 cm^-3 and T_k ~
30 K. Continuum emission at 3.4 mm, 2.7 mm and 1.4 mm was mapped to determine
the distribution and amount of HII regions and dust. Column densities derived
from C18^O and 1.4 mm dust continuum fluxes indicate the CO conversion factor
in the center of Maffei 2 is lower than Galactic by factors of ~2-4. Gas
morphology and the clear ``parallelogram'' in the Position-Velocity diagram
shows that molecular gas orbits within the potential of a nuclear (~220 pc)
bar. The nuclear bar is distinct from the bar that governs the large scale
morphology of Maffei 2. Giant molecular clouds in the nucleus are nonspherical
and have large linewidths. Dense gas and star formation are concentrated at the
sites of the x_1-x_2 orbit intersections of the nuclear bar, suggesting that
the starburst is dynamically triggered.Comment: 50 pages, 14 figures, accepted for publication in Ap
An extragalactic supernebula confined by gravity
Little is known about the origins of the giant star clusters known as
globular clusters. How can hundreds of thousands of stars form simultaneously
in a volume only a few light years across the distance of the sun to its
nearest neighbor? Radiation pressure and winds from luminous young stars should
disperse the star-forming gas and disrupt the formation of the cluster.
Globular clusters in our Galaxy cannot provide answers; they are billions of
years old. Here we report the measurement of infrared hydrogen recombination
lines from a young, forming super star cluster in the dwarf galaxy, NGC 5253.
The lines arise in gas heated by a cluster of an estimated million stars, so
young that it is still enshrouded in gas and dust, hidden from optical view. We
verify that the cluster contains 4000-6000 massive, hot "O" stars. Our
discovery that the gases within the cluster are bound by gravity may explain
why these windy and luminous O stars have not yet blown away the gases to allow
the cluster to emerge from its birth cocoon. Young clusters in "starbursting"
galaxies in the local and distant universe may be similarly gravitationally
confined and cloaked from view.Comment: Letter to Natur
The drivers of Chinese CO2 emissions from 1980 to 2030
China's energy consumption doubled within the first 25 years of economic reforms initiated at the end of the 1970s, and doubled again in the past 5 years. It has resulted of a threefold CO2 emissions increase since early of 1980s. China's heavy reliance on coal will make it the largest emitter of CO2 in the world. By combining structural decomposition and inputâoutput analysis we seek to assess the driving forces of China's CO2 emissions from 1980 to 2030. In our reference scenario, production-related CO2 emissions will increase another three times by 2030. Household consumption, capital investment and growth in exports will largely drive the increase in CO2 emissions. Efficiency gains will be partially offset the projected increases in consumption, but our scenarios show that this will not be sufficient if China's consumption patterns converge to current US levels. Relying on efficiency improvements alone will not stabilize China's future emissions. Our scenarios show that even extremely optimistic assumptions of widespread installation of carbon dioxide capture and storage will only slow the increase in CO2 emissions
The Long-Term Variability of the X-Ray Sources in NGC 6946 and NGC 4485/4490
We analyze data from five Chandra observations of the spiral galaxy NGC 6946
and from three Chandra observations of the irregular/spiral interacting galaxy
pair NGC 4485/4490, with an emphasis on investigating the long-term variability
exhibited by the source populations. We detect 90 point sources coincident with
NGC 6946 down to luminosities of a few times 10^36 erg/s, and 38 sources
coincident with NGC 4485/90 down to a luminosity of ~1x10^37 erg/s. Twenty-five
(15) sources in NGC 6946 (NGC 4485/90) exhibit long-term (i.e., weeks to years)
variability in luminosity; 11 (4) are transient candidates. The single
ultraluminous X-ray source (ULX) in NGC 6946 and all but one of the eight ULXs
in NGC 4485/90 exhibit long-term flux variability. Two of the ULXs in NGC
4485/90 have not been identified before as ultraluminous sources. The
widespread variability in both systems is indicative of the populations being
dominated by X-ray binaries, and this is supported by the X-ray colors of the
sources. The distribution of colors among the sources indicates a large
fraction of high-mass X-ray binaries in both systems. The shapes of the X-ray
luminosity functions of the galaxies do not change significantly between
observations and can be described by power laws with cumulative slopes ~0.6-0.7
(NGC 6946) and ~0.4 (NGC 4485/90).Comment: 26 pages, 9 figures, 15 tables - to appear in the August 2008 issue
of ApJS - new version corrects a few typo
A Chandra X-ray Analysis of Abell 1664: Cooling, Feedback and Star Formation in the Central Cluster Galaxy
The brightest cluster galaxy (BCG) in the Abell 1664 cluster is unusually
blue and is forming stars at a rate of ~ 23 M_{\sun} yr^{-1}. The BCG is
located within 5 kpc of the X-ray peak, where the cooling time of 3.5x10^8 yr
and entropy of 10.4 keV cm^2 are consistent with other star-forming BCGs in
cooling flow clusters. The center of A1664 has an elongated, "bar-like" X-ray
structure whose mass is comparable to the mass of molecular hydrogen, ~ 10^{10}
M_{\sun} in the BCG. We show that this gas is unlikely to have been stripped
from interloping galaxies. The cooling rate in this region is roughly
consistent with the star formation rate, suggesting that the hot gas is
condensing onto the BCG. We use the scaling relations of Birzan et al. 2008 to
show that the AGN is underpowered compared to the central X-ray cooling
luminosity by roughly a factor of three. We suggest that A1664 is experiencing
rapid cooling and star formation during a low-state of an AGN feedback cycle
that regulates the rates of cooling and star formation. Modeling the emission
as a single temperature plasma, we find that the metallicity peaks 100 kpc from
the X-ray center, resulting in a central metallicity dip. However, a
multi-temperature cooling flow model improves the fit to the X-ray emission and
is able to recover the expected, centrally-peaked metallicity profile.Comment: 15 pages, 13 figure
Warm Molecular Gas in Dwarf Starburst Galaxies: CO(3-2) Observations
Eight dwarf starburst galaxies have been observed with the Caltech
Submillimeter Observatory (CSO) telescope in the CO J= 3 - 2 transition. The
galaxies observed are He 2-10, NGC 5253, NGC 1569, NGC 3077, Haro 2, Haro 3, II
Zw 40 and Mrk 86; all but the last two are detected. The central regions of He
2-10 and NGC 5253 were mapped and a CO(2-1) spectrum of NGC 5253 was obtained.
The error weighted mean CO(3-2)/CO(1-0) ratio of the detected galaxies is
0.600.06, which is virtually identical to what is found for starbursts in
the nuclei of nearby spirals, and suggests that the molecular gas is optically
thick, warm (T20 K), and moderately dense (). The CO(3-2)/CO(1-0) ratio peaks at or close to the starburst in all
cases. CO emission does not appear to be optically thin in these dwarfs,
despite the low metallicity and intense radiation fields, which is probably
because in order for CO to exist in detectable amounts it must be
self-shielding and hence optically thick. Physical properties of the molecular
clouds in these dwarf starbursts appear to be essentially the same as nearby
spiral nuclei, with the possible exception that CO is more confined to the
cloud cores.Comment: 21 pages, 8 figures; Accepted for publication by the Astronomical
Journa
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